In medicine, the display of vessels, usually blood vessels, by means of diagnostic imaging methods is called angiography. To this end, a contrast agent, that is, a substance that increases the image contrast or makes it particularly visible in the selected examination method, is often injected into the vessel. The vessel interior filled with the contrast agent then stands out in the image of the recorded body region. The resulting image is called an angiogram.
A distinction is made between different angiographies, depending on the underlying medical imaging method.
In the case of optical angiography, a video recording is generally made of the optical dye front, which is passing through, of an optically detectable contrast agent which was injected in the form of a bolus into the vessel to be examined. It can only measure changes in the blood flow that are close to the surface. A special case of optical angiography is the so-called fluorescence angiography, in which there is photographic representation of the blood vessels with the aid of fluorescent dyes. U.S. Pat. No. 7,580,185 and U.S. patent application publications 2008/0013166, 2010/0097618 and 2011/0168914 describe such a video fluorescence angiography method and a microscope system for carrying out the method. A corresponding system is marketed by Carl Zeiss Meditec AG under the name IR800.
Optical angiography can be carried out intraoperatively without much effort and with almost no restrictions on the number of times. It supplies an image of the shallow vessels in the observation region, even in real time if required, and makes it possible to estimate blood flow changes. The perfusion cannot be quantified in absolute terms on its own, that is, without knowledge of further parameters. This means that it proves impossible to determine absolute values or absolute difference values for the flow speed and/or the volumetric flow rate, that is, the blood volume that passes through a cross section within a unit time, in (relatively large) vessels. However, the blood flow is a very important parameter in, for example, vascular—i.e. relating to blood vessels—neurosurgery.
Computed tomography angiography (CTA) uses modern multi-row computed tomography. Computed tomography is the computer-based evaluation of a multiplicity of X-ray recordings of an object, recorded from different directions, in which the non-captured volume structure is reconstructed afterward in order to produce a three-dimensional image. For diagnostic purposes, two-dimensional slice images are generated from the three-dimensional image and displayed on a monitor. It is possible to display vessels after administering a contrast agent. The functionality of a computed tomography scanner was described in GB 1283915 A for the first time.
The medical imaging method CTA not only affords the possibility of displaying the entire examined vessel structure in three dimensions, but it also makes it possible to calculate the absolute value of the volume flow, or equivalently the absolute value of the volumetric flow rate, at every point in the vessel system. After injecting a small amount of a contrast agent, which generally contains iodine, into the vascular system of the patient, the distribution of said contrast agent in the tissue is recorded using repeated scans with an overall duration of approximately 40 seconds. Here, the X-ray computed tomography scanner produces many slice images of the tissue that are successive in time with the aid of X-ray radiation. From this, a computer can calculate how long the contrast agent takes to be distributed. A method and an arrangement for a spatially resolved calculation of the absolute value of the blood flow, i.e. the flow speed and/or the volumetric flow rate of the blood through the examined vessels, are described, for example, in U.S. Pat. No. 6,373,920 B1. Information in respect of the functionality of the image evaluation is also gathered from the page http://www.innovations-report.de/html/berichte/medizin_gesundheit/bericht-25207. html.
Magnetic resonance angiography or, equally, magnetic resonance imaging (MRI) is based on very strong magnetic fields and alternating electromagnetic fields in the radiofrequency range, by means of which specific atomic nuclei are excited in the body by resonance which then induce electric signals in a detector. In order to determine the location of the respective atomic nuclei, a spatially dependent magnetic field (gradient magnetic field) is applied and thus precise three-dimensional imaging is made possible. Different relaxation times for different tissue types are an important basis for the image contrast. Additionally, the different content of hydrogen atoms in different types of tissue (for example, muscle, bone) also contributes to the image contrast. Administering a contrast agent in particular also makes it possible to display blood vessels. For assessments and for making diagnoses it is necessary for the data records, recorded as three-dimensional data records, to be displayed as two-dimensional images on the monitor. Nuclear magnetic resonance is used synonymously with magnetic resonance imaging. The abbreviation MRI, which can also be found, comes from the English words magnetic resonance imaging. A device and a method for carrying out MRI are described in, for example, U.S. Pat. No. 4,707,658.
The medical imaging method MRI also makes it possible to calculate the absolute value of the volumetric flow rate at each point in the vessel system. A method and an arrangement for quantitatively measuring the perfusion are described in, for example, United States patent application publication 2008/0119720 A1. At the address http://www.vassolinc.com/product.cfm, VasSol offers software called NOVA (Non-invasive Optimal Vessel Analysis) for quantifying the blood flow; the software uses MRA data for the calculation.
The arrangements or devices for carrying out the three-dimensional imaging methods CTA and MRI have a comparatively large design. A computed tomography scanner in the traditional C-arm design comprises two huge arms, in which an X-ray source and a CT detector lie opposite one another and circle the body of the patient. An instrument of this type is described in, for example, EP 0244596 A1. Furthermore, annular computed tomography scanners are known; here the patient is moved into the interior thereof on a couch. The X-ray source circles the patient within the ring. By way of example, an arrangement of this type is described in U.S. Pat. No. 6,373,920 B1. A magnetic resonance imaging scanner usually also has an annular design. By way of example, an arrangement of this type is shown and described in United States patent application publication 2008/0119720 A1.
As a result of the large spatial requirements, the long measurement duration and the lack or restricted accessibility to the examined tissue for the surgeon or operator and assisting medical staff during operations, such instruments or methods may generally only be used pre-operatively, or in the operating theater, only with great effort and/or intermittently with long time intervals.
Although absolute-value measuring flow probes, like, for example, an ultrasound Doppler anemometer, laser Doppler anemometer (LDA) and inductive or capacitive flow sensors, are suitable for contactless measurement of flow or particle speeds, that is, for determining the blood flow, and also supply absolute values in the case of appropriate calibration, such probes can only be used at points.